fix: Use webpack -o instead of --output

Currently the build/test fails with the following output:

```
Test webextension-polyfill bundled with webpack
===============================================
[webpack-cli] Unknown argument: --output
[webpack-cli] Unknown argument: /tmp/webpack-bundle.js
? Which flags do you want to use? …
✔ --entry: The entry point(s) of your application e.g. ./src/main.js
✔ --config: Provide path to a webpack configuration file e.g. ./webpack.config.js
✔ --color: Enable/Disable colors on console
✔ --merge: Merge two or more configurations using webpack-merge e.g. -c ./webpack.config.js -c ./webpack.test.config.js --merge
✔ --progress: Print compilation progress during build
✔ --help: Outputs list of supported flags
✔ --output-path: Output location of the file generated by webpack e.g. ./dist/
```

Regressed by:
webpack/webpack-cli#1855
webpack/webpack-cli@2f36b9d

The `-o` flag is compatible with both the old and new flag name.

scripts/run-module-bundlers-smoketests.js

@@ -8,7 +8,7 @@ Test webextension-polyfill bundled with webpack
 ===============================================`);
 
 process.env.TEST_BUNDLED_POLYFILL = "/tmp/webpack-bundle.js";
-result = shell.exec(`webpack --mode production --entry ./test/fixtures/bundle-entrypoint.js --output ${process.env.TEST_BUNDLED_POLYFILL}`).code ||
+result = shell.exec(`webpack --mode production --entry ./test/fixtures/bundle-entrypoint.js -o ${process.env.TEST_BUNDLED_POLYFILL}`).code ||
   shell.exec("npm run test").code || result;
 
 console.log(`